LUSAKA (Reuters) - President Edgar Lungu was leading in Zambia's presidential election on Monday, with 85 percent of the constituencies counted, but his main rival demanded a recount in a key district, citing irregularities.

Lungu faces a stiff challenge from United Party for National Development (UPND) leader Hakainde Hichilema, who accuses him of running the economy down, a charge the president has rejected.

With 50.14 percent, Lungu was ahead of Hichilema, with 47.7 percent, after results were collated from 132 of 156 constituencies in Aug. 11 voting, the Electoral Commission of Zambia (ECZ) told a news conference.

But Hichilema told a separate media briefing his party wanted a recount of votes in Lusaka district "for the sake of free, fair, credible and transparent elections".

"The question is will the elections be defined as free and fair, transparent and credible in this environment? My answer is no," Hichilema said.

"Zambia needs to remain peaceful. Anybody seeking political office wants to make sure that they take over a country that is peaceful and stable so that you can implement your vision."

The winner of the presidential election in one of the most stable democracies in Africa must get more than half the vote, failing which the top two candidates face a re-run.

The UPND said on Saturday that data from its own parallel counting system showed Hichilema beating Lungu "with a clear margin", based on about 80 percent of votes counted.

All parties have access to the raw voting data and may add up the results faster than the national commission.

The ECZ had hoped to have final results from the elections - in which Zambians also chose members of parliament, mayors and local councillors and decided on proposed constitutional changes by early Sunday.

But it said the process had been lengthened by a large voter turnout, now at 56.22 percent, far above 32 percent last year, when Lungu won an election to replace Michael Sata, who died in office.

The commission and Lungu's Patriotic Front have both rejected the UPND's charges that some officials were working to manipulate results to help the ruling party.

One of its officials accused Hichilema of making inflammatory statements.

"Our main concern is that Mr Hichilema has decided to take his frustrations to a criminal level," said Given Lubinda, a member of the parliament dissolved ahead of the vote.

Supporters of the two main parties clashed over rising unemployment, mine closures, power shortages and soaring food prices after weak global prices hit exports of copper, the mainstay of the economy.